An e-book (also: eBook, ebook), sometimes called an electronic book, is an electronic (or digital) equivalent of a conventional printed book. The term has occasionally been used ambiguously to refer to either an individual work in a digital format, or a hardware device used to read books in digital format, more specifically called an e-book device or e-book reader. E-books are an emerging and rapidly changing technology, that can branch to include other formats, such as a online magazines, such as the Grantville Gazette, published by Baen's Books, or digital books designed to be listened to as audio books.

The term e-text is a broader term than e-book, and is also used for the particular case of data in ASCII text format, rather than books in proprietary file formats. It also includes the academic e-text, which commonly contains components such as facsimile images, apparatus criticus, and scholarly commentary on the work from one or more editors specially qualified to edit the author or work in question.

An e-book is commonly bundled by a publisher for distribution (as an e-book, an ezine, or an Internet newspaper), whereas e-text is distributed in plain text on the Web, or in the case of academic works, in the form of discrete media such as compact discs. Metadata relating to the text are sometimes included with etext (though it appears more frequently with e-book). Metadata commonly include details about author, title, publisher, and copyright date; less common are details regarding language, genre, relevant copyright conventions, etc.

Advantages

Text can be searched, except when represented in the form of images.
Take up little space.
Hundreds (or thousands) may be carried together on one device.
Approximately 500 average e-books can be stored on one CD (equivalent to several shelves' worth of print books)
Because they take up little space, e-books can be offered indefinitely, with no 'out of print' date, allowing authors to continue to earn royalties indefinitely (copyright law permitting), and allowing readers to find older works by favorite authors.
E-books may be read in low light or even total darkness, with a back-lit device.
Type size and type face may be adjusted. However, enlarging e.g. a PDF document magnifies the text but preserves the original layout and spacing; a practical limit on zooming follows from the requirement to keep a text column within the width of the screen (otherwise horizontal scrolling would be needed during and after reading each line, which would be very cumbersome). However, tagged PDFs can be reflowed in Acrobat 6 and 7, eliminating the horizontal-scrolling problem in zoomed PDFs. For more on zooming in, see Electronic maps.
Can be used with text-to-speech software.
Readily reformatted for independent platforms.

Disadvantages

From the user's point of view:
Can be incompatible with new or replacement hardware or software
Require care in handling and storage of the files, to avoid damage or loss
Can Phone Home to track readers and reading habits
Can restrict printing
Devices are for most prohibitively expensive and can be lost, stolen, and damaged like any electronic device
Batteries run out. Books don't require batteries
Their average price is considered by many to be too expensive when compared to the print edition.

from : en.wikipedia.org